Time-domain reflectometry or TDR is a method for determining and analysing run lengths and reflection characteristics of electromagnetic waves and signals.
In a TDR-based fill-level measuring instrument, a low-energy electromagnetic pulse is generated by the electronics of the sensor, coupled into a conductor (also referred to as a probe) and guided along this probe. The probe is generally a coaxial waveguide, a metal rod or a steel cable.
If this microwave pulse then hits the surface of the medium to be measured, part of the pulse is reflected at said surface and returns along the probe to the electronics, which then calculates the fill level from the time difference between the transmitted pulse and the received pulse (in the nanosecond range).
The sensor can output the calculated fill level as a continuous analogue signal or a switch signal. An advantage of this method is that the measurement result is scarcely influenced by the properties of the medium to be measured, for example density, conductivity and dielectric constant, or by the environmental conditions, for example pressure and temperature, and that no interference-prone moving parts are required.
The probe of the TDR sensor ensures that the signal reaches the filling material undisturbed. Liquids, bulk materials and interfaces in liquids are measured using this measuring method.
Not least for geometric reasons, however, the measuring accuracy of the fill-level measuring instrument can be impaired in the region of the probe end which is turned towards the filling material.